Tissue may be destroyed, ablated, or otherwise treated using thermal energy during various therapeutic procedures. Many forms of thermal energy may be imparted to tissue, such as radio frequency (RF) electrical energy, microwave electromagnetic energy, laser energy, acoustic energy, or thermal conduction. In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) for the purpose of ablating predictable volumes of tissue with minimal patient trauma. RF ablation of tumors is currently performed using one of two core technologies.
The first technology uses a single needle electrode, which when attached to a RF generator, emits RF energy from the exposed, uninsulated portion of the electrode. This energy translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. In theory, RF ablation can be used to uniformly sculpt the volume of necrosis to match the extent of the tumor. By varying the power output and the type of electrical waveform, it is theoretically possible to control the extent of heating, and thus, the resulting ablation. The diameter of tissue coagulation from a single electrode, however, is limited by heat dispersion. As a result, multiple probe insertions have been required to treat all but the smallest lesions. This considerably increases treatment duration and requires significant skill for meticulous precision of probe placement.
The second technology utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. U.S. Pat. No. 6,379,353 discloses such a probe, which is commercially available as the LeVeen Needle Electrode. This probe comprises a cannula having a needle electrode array, which is reciprocatably mounted within the cannula to alternately deploy the electrode array from the cannula and retract electrode array within the cannula. The individual electrodes within the array have spring memory, so that they assume a radially outward, arcuate configuration as they are deployed from the cannula. In general, a multiple electrode array creates a larger lesion than that created by a single needle electrode.
Whichever technology is utilized, increasing generator output has been unsuccessful for increasing lesion diameter, because an increased wattage is associated with a local increase of temperature to more than 100° C., which induces tissue vaporization and charring. This then increases local tissue impedance, limiting RF deposition, and therefore heat diffusion and associated coagulation necrosis.
To reduce the local temperature, thereby minimizing tissue vaporization and charring, the needle electrode or electrodes can be cooled. With regard to the single needle technology, two coaxial lumens may currently be provided in the needle electrode, one of which is used to deliver a cooled saline (e.g., room temperature or cooler) to the tip of the electrode, and the other of which is used to return the saline to a collection unit outside of the body. See, e.g., Goldberg et al., Radiofrequency Tissue Ablation: Increased Lesion Diameter with a Perfusion Electrode, Acad Radiol, August 1996, pp. 636-644.
Although the circulation of a cooled fluid through the needle electrode provides for a more efficient means for creating a lesion, it requires additional equipment in the form of a pump and collection reservoir. In addition, uniform lesions are not always created even when the needle electrodes are cooled, because the vascular heating sinking effect often pulls heat away from adjacent tissue that is being ablated.
There thus remains a need to provide an improved method, apparatus, and system for cooling tissue during an ablation procedure.